Stable synthetic ester lubricant compositions



3,535,243 STABLE SYNTHETIC ESTER LUBRICANT COMPOSITIONS Tai S. Chao, Homewood, Ill., and Manley Kjonaas, Hammond, Ind., assiguors to Sinclair Oil Corporation, a corporation of New York No Drawing. Filed Aug. 13, 1968, Ser. No. 752,187 Int. Cl. 010m 1/32 US. Cl. 25251.5 12 Claims ABSTRACT OF THE DISCLOSURE A normally liquid synthetic ester base lubricant composition having enhanced oxidation retardation at temperatures greater than 400 F. is disclosed. The composition contains, in addition to the base lubricant, minor amounts of (1) aromatic monoamine having a nonolefinic, non-acetylenic hydrocarbon radical of about 1 to 20 carbon atoms and a non-olefinic, non-acetylenic aromatic hydrocarbon radical of about 6 to 16 carbon carbon atoms attached to the amine group, and (2) diaminonaphthalene or hydrocarbon-substituted diamino naphthalene.

This invention relates to ester-based lubricant compositions containing a novel combination of additive agents. More particularly the present invention relates to ester-based lubricant compositions which exhibit increased oxidation resistance.

Organic compounds, such as lubricating oils, undergo oxidation upon exposure to air. This process is accentuated by elevated temperatures such as occur in engines and other operating machinery. When such organic compositions are used as motor or machinery lubricants, their stability is still further drastically reduced due to their contact with metal surfaces which give up metallic particles into the lubricant. Such abraded or dissolved metals or metal salts appear to act as oxidation catalysts in the lubricant causing the formation of primary oxidation products which in turn cause further degradation of the organic compounds present in the composition. Problems of this nature are encountered in mineral oils but appear to be particularly troublesome in synthetic oleaginous fluids exemplified by esters.

The development of modern high speed jet turbine aircraft engines necessitates the search for lubricants which are resistant to high temperature oxidative degradation. Jet turbines are actuated by the energy of a burning fuel and are used to drive compressors which provide large amounts of air for the burning of the fuel. The combustion of the fuel provides the energy for driving the compressors with the remainder of the useful energy going into the propulsion of the aircraft. Jet turbine bearings are lubricated by pumping a lubricant to the bearings from a reservoir in a closed system. The design of more powerful jet turbines has led to an increase in the lubricant reservoir temperature as well as the temperature of the bearings and various heat sections. This causes greater difliculty in maintaining lubricant stability under the more severe conditions. For ex- 3,535,243 Patented Oct. 20, 1970 ample, a lubricant which will operate satisfactorily at a reservoir temperature of, say, 250 F. may sludge badly, build up viscosity, develop high acidity and corrode metals at a reservoir temperature of, say, 400 F.

Thus, jet turbine service requires a lubricant of superior thermal and oxidation stability as well as satisfactory physical characteristics in terms of viscosity, flash point, volatility, and load carrying properties. To arrive at such a lubricant it has been the general practice to add antioxidants and other additives, such as foam inhibitors, anti-wear agents, etc., to a base fluid of suitable properties.

Numerous oxidation and corrosion inhibitors have been found for use in lubricating compositions and many combinations thereof also have been tested. For instance, antioxidants such as phenothiazine, N-phenyl-u-naphthylamine, S-ethyl 10,10 diphenylphenazasiline, etc., are well known in the art. The known inhibitors, however, are not adequate to provide the superior oxidation resistance now needed. Moreover, the effectiveness of these known inhibitors is greatly impaired in the presence of metal.

It has now been found that the use of a combination of a diaminonaphthalene with an aromatic monoamine provides unexpected and greatly increased resistance to oxidation of normally liquid, synthetic ester lubricants at high temperatures.

The aromatic monoamine component of the invention is soluble in the ester fluid at least to the extent used and can be represented by the following general formula:

wherein Q is a monovalent hydrocarbon radical of l to 20 carbons, preferably 6 to 12 carbon atoms, whose adjacent carbon atoms are no closer than 1.40 A. (i.e., a non-olefinic, non-acetylenic, monovalent hydrocarbon), and Q is a non-olefinic, non-acetylenic aromatic hydrocarbon radical of 6 to 12 or 16 carbon atoms. Thus Q can be an alkyl group, including cycloalkyl, or an aromatic group. Preferably, both Q and Q are aromatic, and often at least one is a fused ring aromatic, e.g. naphthyl. Q and Q can be substituted with non-interfering substituents such as alkyl, and aryl, and Q and Q can be linked together by means of a non-interfering element such as carbon, sulfur and oxygen. Illustrative of suitable amines are phenothiazine, N-phenyl-u-naphthyl amine; diphenyl amine; p-octyl diphenylamine, p-p' dioctyl diphenylamine; etc.

The diaminonaphthalenes, which constitute one component of the anti-oxidant combination of the present invention, can be unsubstituted diaminonaphthalenes or similar materials which are substituted on one or all of the amine nitrogen atoms and aromatic rings. In any event the additive is soluble in the ester base oil at least in the amounts utilized, and will usually contain up to about 30 or 40 carbon atoms or more. Also the diamino group of these anti-oxidants have either primary or secondary amine nitrogen atoms. Frequently, the substituents on the nitrogen atoms or aromatic rings are hydrocarbon radicals having no olefinic or acetylenic unsaturation. Either the aromatic rings or the hydrocarbon radicals of the diaminonaphthalene additives can be further substituted with non-interfering groups. The hydrocarbon substituents often have up to about 20 or 30 or more carbon atoms and can be, for example, alkyl, aryl or mixed alkyl-aryl groups. Illustrative suitable diaminonaphthalenes include 1,8 diaminonaphtha'lene; 1,5-diaminonaphthalene; 1,4-diaminonaphthalene; the corresponding methyl and octyl-ring substituted naphthalenes; and such diaminonaphthalenes having, for instance, actyl, phenyl or methyl styrenyl groups on one or both of the amino nitrogen atoms.

The lubricant composition of this invention includes as the major component a base oil which is an ester of lubricating viscosity which may be, for instance, a simple ester or compounds having multiple ester groupings such as complex esters, dior other polyesters, and polymer esters. These esters are usually made from monoand poly-functional aliphatic alcohols or alkanols, and aliphatic monoand polycarboxylic or alkanoic acids. Frequently, the alcohols and acids have about 4 to 12 carbon atoms. The reaction product of a mono-functional alcohol and a monocarboxylic acid is usually considered to be a simple ester. A diester is usually considered to be the reaction product of 1 mole of a dicarboxylic acid, say of 6 to carbon atoms, with 2 moles of a monohydric alcohol, or of 1 mole of a glycol, for instance, of 4 to 10 carbon atoms, with two moles of a monocarboxylic acid, e.g. of 4 to 10 carbon atoms. The diesters frequently contain from 16 to 40 carbon atoms.

A complex ester is usually considered to be of the type X-YZYX in which X represents a monoalcohol residue. Y represents a dicarboxylic acid residue and Z represents a glycol residue and the linkages are eester linkages. Those esters, wherein X represents a monoacid residue, Y represents a glycol residue and Z represents a dibasic acid residue are also considered to be complex esters. The complex esters often have 30 to 50 carbon atoms.

Polymer esters or polyester bright stocks can be prepared by direct esterification of dicarboxylic acids with glycols in about equimolar quantities. The polyesterification reaction is usually continued until the product has a kinematic viscosity from about to 200 centistokes at 210 F., and preferably 40 to 130 centistokes at 210 F.

Although each of these products in itself is useful as a lubricant, they are particularly useful when added or blended with each other in synthetic lubricant compositions. These esters and blends have been found to be especially adaptable to the conditions to which turbine engines are exposed, since they can be formulated to give a desirable combination of high flash point, low pour point, and high viscosity at elevated temperatures. In addition, many complex esters have shown good stability to shear. Natural esters, such as castor oil may be employed and also be included in the blends, as may be small amounts of a foam inhibitor such as a methyl silicone polymer, or other additives of lubricant components to provide a particular characteristic, for instance, extreme pressure or load carrying agents, corrosion inhibitors, etc., can be added.

The monohydric alcohols employed in these esters usually contain about 4 to 20 carbon atoms and are generally aliphatic. Preferably the alcohol contains up to about 12 carbon atoms. Useful alkanols include butyl, hexyl, n-octyl, isooctyl and dodecyl alcohols, C oxo alcohols and octadecyl alcohols. C to C branched chain primary alcohols are frequently used to improve the low temperature visocsity of the finished lubricant composition. Alcohols such as n-decanol, Z-ethylhexanol, 0x0

alcohols, prepared by the reaction of carbon monoxide and hydrogen upon the olefins obtainable from petroleum products such as disobutylene and C olefins, ether alcohols such as butyl carbitol, tripropylene glycol monoisopropyl ether, dipropylene glycol monoisopropyl ether, and products such as Tergitol 3A3 which has the formula C H O(CH CH O) H, are suitable alcohols for use to produce the desired lubricant. If the alcohol has no hydrogens on the beta carbon atoms, it is neostructured; and esters of such alcohols are often preferred. In particular, the neo-C alcohol2,2,4-trimethylpentanol-1gives lubricating diesters or complex esters suitable for blending with diesters to produce lubricants which meet stringent viscosity requirements. Iso-octanol and iso-decanol are alcohol mixtures made by the 0x0 process from C C copolymer heptenes. The out which makes up isooctanol usually contains about 17% 3,4- dimethylhexanol; 29% 3,5-dimethylhexanol; 25% 4,5-dimethylhexanol; 1.4% 5,5-dimethylhexanol; 16% of a mixture of 3-methylheptanol and S-ethylheptanol; 2.3% 4-ethylhexanol; 4.3% a-alkyl alkanols and 5% other materials.

Generally, the glycols contain from about 4 to 12 carbon atoms; however, if desired they could contain a greater number. Among the specific glycols which can be employed are 2-ethyl 1,3 hexanediol, 2 propyl 3,3- heptanediol, Z-methyl-1,3-pentanediol, 2-butyl-1,3-butanediol, 2,4-diphenyl-1,3-butanediol, and 2,3-dimesityl-l,3- butanediol. In addition to these glycols, ether glycols may be used, for instance, where the alkylene radical contains 2 to 4 carbon atoms such as diethylene glycol, dipropylene glycol and ether glycol up to 1000 to 2000 molecular weight. The most popular glycols for the manufacture of ester lubricants appear to be polypropylene glycols having a molecular weight of about -300 and 2-ethyl hexanediol. The 2,2-dimethyl glycols, such as neopentyl glycol have been shown to impart heat stability to the final blends. Minor amounts of other glycols or other materials can be present as long as the desired properties of the product are not unduly deleteriously affected.

One group of useful monocarboxylic acids includes those of 8 to 18 or even 24 carbon atoms such as stearic, lauric, etc. The carboxylic acids employed in making ester lubricants will often contain from about 4 to 12 carbon atoms. Suitable acids are described in US. Pat. No. 2,575,195, herein incorporated by reference, and include the aliphatic dibasic acids of branched or straight chain structures which are saturated or unsaturated. The preferred acids are the saturated aliphatic carboxylic acids containing not more than about 12 carbon atoms, and mixtures of these acids. Such acids include succinic, adipic, suberic, azelaic, and sebacic acids and isosebacic acid which is a mixture of a-ethyl suberic acid, a,a-diethyl adipic acid and sebacic acid. This composite of acids is attractive from the viewpoint of economy and availability since it is made from petroleum hydrocarbons rather than the natural oils and fats which are used in the manufacture of many other dicarboxylic acids, which natural oils and fats are frequently in short supply. The preferred dibasic acids are sebacic and azelaic or mixtures thereof. Minor amounts of adipic used with a major amount of sebacic may also be used with advantage.

The ester base oils to which incorporation of the additive combination of the invention is particularly advantageous are the oils commonly ref-errred to as neostructured polyol polyesters, i.e. having more than one ester group. These are the esters of aliphatic carboxylic acids, generally monoalkanoic acids, of about 4 to 12 carbon atoms, and a polyhydric alkanol free of beta hydrogen, i.e. containing no hydrogen on the beta carbon atoms, and including the di(polyhydric alcohol) ethers. The polyhydric alcohol generally contains about 2 to 6 hydroxy groups and about 5 to 20, preferably about 5 to 12 carbonatoms. Illustrative of the alcohols are those having the general formula:

in combination with various diamino-naphthalenes, to Hercolube A, a commercially available, synthetic ester base lube oil, a pentaerythritol ester of a mixture of fatty acids of from 5 to 9 carbon atoms, is shown in Table I. The test data show the volume of oxygen consumed by a 75 gram sample of the lubricant at a temperature of 450 F. and an oxygen flow rate of l ft. /hr., in the presence of 12 p.p.m. of iron (as Nuodex" iron octoate) as a catalyst. The volume of oxygen absorbed was indicated by a Statham gauge and was plotted automatically on a Brown Recorder. The induction period (T is the time in minutes in which comparatively little oxygen is absorbed by the lubricant. The end of the induction period is signalled by a marked increase in the rate of oxygen absorption and V, indicates the milliliters of oxygen absorbed up to that time. T, is the total time required for the lubricant to absorb 2500 ml. of oxygen (V TABLE I.-OXYGEN ABSORPTION TEST DATA 02 absorption test data Sample Wt. Ti, TL, Vi, t. No. Base fluid Additives percent min. min. ml. ml.

1 PAN 1. 5 62 109 410 2,500 2 PAN 2. 74 126 492 2, 500 3 1,8-diamino-naphtha1ene 1. 4 do {LS-diamino-naphthalene 0. 5 131 181 640 2, 500 PAN. 1. 0 265 308 875 2, 500 5 do 1,5-diamino-naphthalene 1. 5

' 1,5-diamino-naphtha1ene 0. 5 150 193 710 2, 500 PAN. 1. 0 257 317 960 2, 500 7 do N 0cty1-1,5diaminonaptha- 1. 5 112 157 696 2, 500

no. N-octyl-l,5-diaminonaphtha- 0 5 8 do Pkelnle. 0 258 297 915 2, 500 N -phenyl-1,8-diaminonaph- 1. 7 265 302 1, 180 2, 500

thalene. N-phenyl-l,S-diaminonaph- 0 7 10 do il lene. 1 0 409 426 985 2, 500 11 do N -a-methy1-styrenated-1,8- 1. 7 153 196 830 2, 500

diaminonaphthalene. Namcthyl-styrenated-l,8- 0 7 12 .do Piiiaminonaphthalene. 0 308 346 1, 120 2, 500 13 do N-octadecyl-1,&diamiuo- 2. 0 100 142 833 2, 500

naphthalene. N-octadecy11,8- diamino- 1 0 14 do I gslrphthalene 1 0 295 335 1, 060 2, 500

propane, pentaerythritol, dipentaerythritol, 2 butyl 2- ethyl-1,3-propanediol, etc. Suitable aliphatic carboxylic acids with which the polyhydric alcohols free of beta hydrogen may be esterified are n-butyric acid, isobutyric acid, valeric acid, isopentanoic acid, caproic acid, isohexanoic acid, n-heptanoic acid, isoheptanoic acid, neoheptanoic acid, n-octanoic acid, isooctanoic acid, pelargonic acid, n-decanoic acid, isodecanoic acid, neodecanoic acid, lauric acid, myristic acid, stearic acid, isostearic acid, etc.

The composition of the present invention contains the amine antioxidant combination in minor amounts sufficient to retard oxidation at high temperatures of the synthetic ester base lubricating oil. Suitable concentration levels of the additives will often be in the range of about 0.1 to 5, preferably about 0.5 to 2, percent of each of the aromatic monoamine and diaminonaphthalene components, based on the total weight of the composition. Other additives, such as antisculf and antifatigue additives, antifoams, etc, can also be advantageously used with the amines of the present invention. The viscosity of the lubrieating" oil containing the additives is preferably less than about 13,000 centistokes at F.

The invention will be better understood by reference to the following example.

EXAMPLE I The antioxidant effects of the addition of varying amounts of N-phenyl-a-naphthylamine (PAN), alone and The advantageous elfects of the antioxidant combinations of the present invention is obvious from the data of Table I. For example: Sample No. 1 containing 1.5 weight percent PAN and required 109 minutes to absorb 2500 ml. of oxygen, and Sample No. 3, containing 1.5 weight percent 1,8-diaminonaphthalene, required 181 minutes to absorb the same volume of oxygen, but in Sample No. 4, where a combination of 0.5 weight percent of 1,-8-diaminonaphthalene and 1.0 weight percent PAN were added to the base fluid, 309 minutes were required to absorb 2500 ml. of oxygen.

Furthermore, a comparison of Samples 2, 13 and 14 shows that 2.0 weight percent PAN required 126 minutes to absorb 2500 ml. of oxygen, and 2.0 weight percent N-octadecyl-1,8-diaminonaphthalene required 142 minutes, but the composition containing 1.0 weight percent of each of these additives required 335 minutes, well over twice as long as either of the additives alone.

EXAMPLE II Table II shows the elfect of the diaminonaphthalene- PAN combination of the invention when used in conjunction with another synthetic ester base oil composition, which composition contains various additives often needed for high temperature lubrication. In these cases PAN and dioctyldiphenylamine (DDP) are used as antioxidants, alone, or with a diaminonaphthalene as a third antioxidant, the tetrabutyl ester of ethylene diamine tetracetic acid (EDTA) is used as a corrosion inhibitor, and the half-amide of a hydrogenated dimer acid is employed as an anti-scuff or extreme pressure agent.

TABLE IL-HIGH TEMPERATURE and acid No. rise, when 1,8-diaminonaphthalene is employed in combination with the PAN-DDP mixture than OXYGEN ABSORPTION TEST DATA O2 absorption test data 1 Wt. Wt. Ti, Tr, Vi, Vt, Sample No. Components percent Third antioxidant percent min. min. m1. ml.

Hereoluloe A 72.22 Hereolube F 2 24. 08 5 Nphenyl-a-naphthylamine 1. O 1 Dioctyldiphenylamine. 1. 00 None 34 84 474 2, 500 Tetrabutyl ester of EDTA. 1. O 23 87 204 2, 500 Haligmide of hydrogenated O.

aci Hercolube A 72. 22 Hercolube F 24. 08 16 N-phenyl-a-naphthylaml 1. 00

--------------------- Diocty1diphenylamine 1. 00 1,8-dia1muonaphthalcuc... 0. 50 348 364 364 2, 500

Tetrabutyl ester of EDTA 1. 50 Halt amide of hydrogenated d" 0.20

ac Hercolube A 72. 22 Hercolube F 24. O8 17 N-phenyl-a-naphthyla 1. 00

Di0ctyldiphenylamine 1. 00 1,8-diammonaphthalene. 0. 20 200 224 1, 535 2, 500

Tetrabutyl ester of EDTA 1. 50 Halt gmido of hydrogenated d 0.20

aci Hercolube A. 72. 22 I-Iercolnbe F 2 24. 08 1 N-phenyl-a-naphthylamin 1.00

8 Diocty1diphenylamine 1.00 1,5-diaminonaphthaleno 0. 50 371 385 2, 030 2, 500

Tetrabutyl ester of EDTA 1. 50 Haltgmide of hydrogenated dimer 0.20

am Hercolube A 72.22 Hercolube F 24.08 19 N -phenyl-a-napht y 1. 00 I Dioetyldiphenylamine 1. 00 1,5-d1an11uonaphthaleue.. 0. 20 284 310 1, 580 2, 500

Tetrabutyl ester of EDTA 1. 50 Haltgmide of hydrogenated dimer 0.20

am Hercolube A 72. 22 Hercolube F 24. O8 20 JN-phenyM-naphthy 1. 00

--------------------- Dioctyldiphenylamine 1.00 -oetyl-1,S-diaminonaphtha- 0. 50 328 338 2, 3 0 2, 5 0

Tetrabutyl ester of EDT 1. 50 lone. Haligmide of hydrogenated 0. 20

acr Hercolube A 72.22 Hercolube F 24.08 21 N-phenyl-a-naphthylarm 1. 00 t Dioetyldiphenylamine. 1. O0 N-octadecyl-l,8-d1am1nonaph- 1. 0 228 247 1, 940 2, 500

Tetrabutyl ester of EDTA 1. 50 thalene. Haligmide of hydrogenated dimer 0. 20

am Hercolube A 73. 72 22 Hercolube F 2 24. 58

--------------------- Tetrabutyl ester of EDT 1. 00 1,8-d1am1nonaphthalene.. 0. 50 47 96 312 2, 500

Halt gmide of hydrogenated dimer 0. 20

am Hercolube A 73. 72 24. 58

1.00 1,5-d1aminonaphthalene 0.50 71 114 364 2, 500 Half gmide of hydrogenated dimer 0. 20 J acr 1 Same conditions as those in Table I.

EXAMPLE III Table III shows in yet another manner the antioxidant efiectiveness of the additive combination of the present invention when used in the same synthetic ester base lubricating composition as was used in Example II. The type 1% bearing rig test was conducted under conditions specified by Navy specification MIL.L23699, which conditions have been shown to correlate with actual jet engine performance.

TABLE III.TYPE 1% BEARING RIG TEST DATA Wt. percent Sample No 24 25 Composition:

Hereolubo A 71. 99 72. 09

Hercolube F 24. 01 24.21

N-phenyl-a-naphthylamine (PAN)... 1.00 1.00

Dioctyldiphenylamine (DDP) 1.00 1. 00

Tetrabutyl EDTA 1. 1. 50

Half amide of hydrogenated dimer acid. 0.20 0.20

1, 8 diaminonaphthalene 0. 30 None Bearing rig test results:

Viscosity increase, KV/lOO, percent 23 108 Final Acid No 1. 32 7. 42

Conditions: duration, 100 hrs.; bearing temp, 500 F.; oil in temp., 350 F.; sump temp., 390 F.; bearing speed, 10,000 r.p.m.; bearing load, 500 1b.; oil flow, 600 m1./m1n.; air flow; 9,900 m1./1nin.

The results reported in Table III show significantly better performance, in terms of retarding viscosity increase 2 A dipentaerythritol ester of a mixture of C5-C9 fatty acids.

when the latter is used alone. Thus, viscosity increase is more than four times, and the acid No. increase more than six times, as great when 1,8-diaminonaphthalene was excluded from the lubricant composition.

What is claimed is:

1. A normally liquid, synthetic ester base, high temperature lubricant composition comprising a major amount of a carboxylic acid ester base oil of lubricating viscosity having dissolved therein minor amounts, sufficient to retard oxidation of the composition of an aromatic monoamine having the formula:

3. The comopsition of claim 1 wherein said member is selected from the group consisting of 1,8-diaminonaphthalene, N-phenyl-l,8-diaminonaphthalene, and N-methylstyrenated-1,8-diaminonaphthalene.

4. The composition of claim 1 wherein the aromatic monoamine is selected from the group consisting of N- phenyl-u-naphthylamine and dioctyldiphenylamine.

5. The composition of claim 3 wherein the aromatic amine is selected from the group consisting of N-phenyla-naphthylamine and dioctyldiphenylamine.

6. The composition of claim 5 wherein each of said aromatic monoamine and said member is present in an amount of about 0.5 to 2 percent by weight of the composition.

7. The composition of claim 1 wherein the base oil is an ester of a monoalkanoic acid of about 4 to 12 carbon atoms and a polyhydric alcohol having no hydrogen on beta carbon atoms, 2 to 6 hydroxyl groups and about 5 to 12 carbon atoms.

8. The composition of claim 7 wherein the polyhydric alcohol is pentaerythritol or dipentaerythritol.

9. The composition of claim 7 wherein said member is selected from the group consisting of 1,8-diaminonaphthalene, N-phenyl-l,8-diaminonaphthalene, and N-methylstyrenated-1,8-diaminonaphthalene.

10. The composition of claim 7 wherein the aromatic monoamine is selected from the group consisting of N- phenyl-a-naphthylamine and dioctyldiphenylamine.

11. The composition of claim 7 wherein said member is selected from the group consisting of 1,8-diaminonaphthalene, N-phenyl-1,8-diaminonaphthalene, and N-methylstyrenated-l,S-diaminonaphthalene and the aromatic monoamine is selected from the group consisting of N- phenyl-u-naphthalarnine and dioctyldiphenylamine.

12. The composition of claim 11 wherein each of said aromatic monoamine and said member is present in an amount of about 0.5 to 2 percent by Weight of the composition.

References Cited UNITED STATES PATENTS 2,771,367 11/1956 Thompson et al. 252-403 X 3,247,111 4/1966 Oberright et al.

3,282,840 11/1966 Foster et al.

3,347,791 10/1967 Thompson et al. 252 X PATRICK P. GARVIN, Primary Examiner W. I. SHINE, Assistant Examiner U.S. Cl. X.R. 25250, 401 

